In recent years, needs for optical functional film are increasing. As a concrete example of the optical functional film, for example, a light reflecting film having a high reflectance, without metal plating, by laminating layers of different refractive index in a multi-layer is mentioned. And, an optically isotropic film having a low photoelastic coefficient suitable for liquid crystal display applications can be mentioned.
For example, in Patent reference 1, a light reflecting film in which a copolymerized polyester resin composition is laminated to a polyethylene naphthalate resin composition (hereafter, PEN), and in Patent reference 2, a multi-layer optical film in which PEN and a copolymerized polyester resin composition are laminated, are proposed. However, in the polyester resin compositions described in Patent references 1 and 2, since polyesters different in Tg are laminated with each other, they are inferior in processability. Furthermore, the polyester described in Patent references 1 and 2 are high in photoelastic coefficient, and cannot be used for a liquid crystal display or the like.
As a copolymerized polyester excellent in thermal resistance which can be used for optical applications, a polyester having cyclic acetal skeleton can be mentioned. For example, in Patent references 3 and 4, at copolymerizing a dicarboxylic acid, a diol or the like having a cyclic acetal skeleton, production methods in which ester exchange reaction is carried out under a high pressure or under a nitrogen atmosphere are described. However, in said production methods, the diol having cyclic acetal skeleton cannot be reacted efficiently, and there are problems such as that, in a condensation polymerization reaction, the diol component having cyclic acetal skeleton is scattered, or a gelation occurs during the polymerization.
In Patent references 5 and 6, it is described that a polymer of which yellowness index is low and sublimate is little can be obtained by copolymerizing an alicyclic component to a polyester resin in presence of a titanium compound in an oligomerization step such as ester exchange. However, although the gelation can considerably be prevented by using the titanium compound, after producing the polyester resin, in a later step such as producing a film for a long time, since an opening or the like of the alicyclic component occurs by thermal degradation, it is insufficient as a prevention of gelation.
In addition, in case where the polyester resin composition containing a spiroglycol shown in these Patent references is used to a multi-layer laminate film constituted together with a polyethylene terephthalate (hereafter, PET) resin layer, since its glass transition temperature (hereafter, Tg) is higher than that of ordinary PET, at molding into a film, a laminate nonuniformity or the like may arise and it is inferior in processability. On the other hand, in case where the Tg is controlled approximately to that of PET by decreasing the content of spiroglycol, it is impossible to sufficiently lower its refractive index, and the light reflectance of the obtained multi-layer laminate film becomes low.
In addition, in Patent reference 7, a production method for obtaining a polyester resin in which a phosphorus-based antioxidant, etc., is compounded by blending/kneading to a polyester resin composition having a cyclic acetal skeleton is shown. However, it cannot deal well with thermal degradation during the polyester resin production, and its effect is insufficient.
[Patent reference 1]: JP-2000-141567A
[Patent reference 2]: JP-H9-506837A (national publication of translated version) (Columns 2 to 6)
[Patent reference 3]: JP-2004-67829A (Columns 1 to 9)
[Patent reference 4]: JP-2003-183422A
[Patent reference 5]: JP-2005-314643A (Columns 1 to 13)
[Patent reference 6]: JP-2006-225621A (Columns 1 to 19)
[Patent reference 7]: JP-2004-67830A (Columns 1 to 4)